Excessive current may cause significant damage to an electronic circuit. When a semiconductor measurement apparatus tests short-circuit limit characteristics of a device under test (PUT) such as an IGBT (insulated gate bipolar transistor), it gradually increases the voltage between the collector and emitter of the transistor to test the state of short-circuit or breakdown between the collector and emitter. When the DUT is shorted, excessive current flowing through the semiconductor measurement apparatus may damage the semiconductor measurement instrument. When a motor is shorted by accident, the control circuit of the motor may be seriously damaged by excessive current. Because excessive current may damage not only a shorted circuit but also its associated circuits and control circuits, some appropriate protection circuit is required for protecting such circuits from damage due to excessive current. A typical circuit breaker using an electromagnetic relay is not suitable for rapid interception of such excessive current.
A conventional circuit breaker employing a rapid fuse or a super rapid fuse can rapidly intercept such excessive current but such a special fuse is an expensive solution to the problem because such a fuse can be used only one time. In particular, the short-circuit test of a semiconductor device is repeated many times, so that such a fuse is not cost-effective for protection of the circuit.
Another rapid breaker apparatus is disclosed in U.S. Pat. No. 5,455,502 of Kato et al. This apparatus employs an IGBT including N (integer) IGBT current paths coupled in parallel and inserted through a load current path. Each IGBT receives a control signal at its gate and the N IGBT current paths are normally turned on, so that only one N-th of the load current flows through each of the IGBT current paths and thus this apparatus can flow a large current. If each of the IGBT current paths is structured by M (integer) series-coupled IGBTs, the permissible voltage limit of the apparatus is M times as high as that of one IGBT. A current-sensing resistor is inserted through the load current path and when excessive current is detected, the IGBTs are rapidly turned off. This IGBT breaker apparatus can be used many times and is therefore very inexpensive.
The voltage-to-current characteristic of one IGBT is slightly different from that of another. FIG. 1 represents the voltage-to-current characteristics of two IGBTs which may be coupled in series. The horizontal axis represents the collector-to-emitter voltage Vce and the vertical axis represents the collector current Ic of the IGBTs. The curve 48 represents the characteristic of the lower one of the two series-coupled IGBTs and the curve 50 represents the characteristic of the upper one of them. It should be noted that the upper-lower relation of the IGBTs may be reversed. In a typical state the IGBTs are conductive and the lower IGBT has an operational point 52 and the upper IGBT has an operational point 54. When some abnormality such as a short-circuit accident occurs and the load current is increased, the current flowing the series-coupled IGBTs is also increased. If the current reaches Is56, the lower IGBT is saturated at the operational point 56 but the upper IGBT is not saturated at the operational point 58.
If one of the two series-coupled IGBTs is saturated but the other thereof is not saturated, the voltage balance between the two IGBTs is collapsed, so that the collector-to-emitter voltage of the saturated IGBT is rapidly raised and this saturated IGBT may be destroyed. This problem may similarly occur in the case where three or more IGBTs are coupled in series or transistors other than IGBTs such as power MOSFETs are used.
Furthermore, as the IGBTs which are coupled in parallel may have different characteristics, the current of one IGBT may be different from the current of another IGBT. As the load current is increased, the current balance among the parallel-coupled IGBTs may be broken, so that a particular IGBT through which the largest current flows may be damaged.
FIG. 2 represents the operation of the IGBT breaker apparatus disclosed in U.S. Pat. No. 5,455,502. The horizontal axis represents time and the vertical axis represents current flowing through the IGBT breaker, or load current. When the IGBT breaker apparatus is conductive, the load current is normally kept at a stable value Is. When the current path of the load is shorted to ground by accident at the time point t0, the load current is rapidly increased along some slope which is determinedly the time constant condition of the current path. When the current reaches the value Isat at the time point t1, the voltage across the current-sensing resistor also reaches an upper limit value, so that the IGBT breaker apparatus is turned off and the current rapidly decreases.
At the time point t1 the slope polarity of the current is abruptly changed, so that a fast spike noise or a transition pulse is generated and may cause damage to the load and its associated circuits.
What is desired is that an improved high speed, large-current power control apparatus will be provided to overcome the above-mentioned problems.